Breast cancer screening has been recommended for many decades, particularly for women over the age of fifty. The combination of early detection and improved therapy of breast cancer in the U.S. has resulted in a significant reduction in breast cancer mortality, with similar reductions being observed in other countries. Despite the success of screening mammography, however, it is also recognized that mammography is a less than perfect screening method. The limitations of mammography are particularly evident when it is used on women having mammographically-dense breasts. It has been shown that the sensitivity of mammography decreases with increasing mammographic density, and is less than fifty percent for women with an extremely dense breast pattern on a mammogram.
The reduction of sensitivity of mammography with the increase of mammographic density is compounded by the fact that increased breast density is a significant risk factor for breast cancer. Given that a dense breast pattern is more characteristic of younger women, this factor significantly diminishes the value of mammography in the screening of young women who have a high familial or genetic risk of breast cancer.
A second major limitation to screening mammography lies in the evaluation of women at high risk of breast cancer. Numerous studies have demonstrated that, when performed on women with a high genetic risk of breast cancer, mammography has a sensitivity of between about 33 and about 43 percent. Most of these studies have been performed with women with an average age of forty, so part of the explanation for the poor performance of mammography in these studies may be due to the presence of dense breast patterns in a significant percentage of the mammographic images.
A possible solution to the problem of the detection of breast lesions in dense breast tissue is to use ultrasound-based techniques with such patients. Ultrasound (US) techniques are attractive for supplemental screening because they are widely available, well-tolerated by patients, and involve no exposure of the patients to radiation. However, while supplemental US screening uncovers more occurrences of breast cancer, it also substantially increases the risk of a “false positive” cancer finding and unnecessary biopsy. Hence, the use of whole-breast ultrasound as a sole identifier of breast malignancies is questionable. Even in combination with mammography, the two anatomical techniques have significant limitations. It would be of considerable benefit to provide another complementary method that offers functional information about lesions available from the results of the US screening. Such a method would significantly reduce the number of “false positive” cases, and allow the radiologist to evaluate those lesions that demonstrate both a functional and anatomical abnormality.
Over the last five years, several nuclear medicine-based technologies have been developed that have application in breast imaging. Included in these are positron emission mammography (“PEM”) and molecular breast imaging (“MBI”). In PEM the breast is compressed between two opposing detectors and the 511 keV gamma rays emitted by a positron-emitting radiopharmaceutical, such as F-18 fluoro-deoxyglucose, for example, are detected by coincidence imaging between the two opposing detectors. The PEM images provide an image of glucose utilization by breast tissue and have been shown to be capable of detecting small cancers in the breast. Unlike anatomical techniques such as mammography and ultrasound, PEM is not influenced by dense breast tissue.
The second nuclear medicine-based technique is MBI. This technology employs one or two small gamma cameras. The breast is compressed between a camera and a compression paddle, or between two gamma cameras, and radiation emitted by a single-photon radiopharmaceuticals, such as Tc-99m sestamibi, is detected after collimation. MBI is a planar imaging technique without tomographic capability; however, information from two opposing gamma cameras can be used to calculate the true depth of a functional abnormality in the MBI images. The MBI system has been shown to have a very high sensitivity (for example in some cases greater than ninety percent) for the detection of lesions smaller than ten millimeters across. In addition, it has been found that, in some cases, MBI can detect three times as many cancer occurrences as digital and analog mammography in asymptomatic women at increased risk of breast cancer.
Beyond sensitivity differences, technologies that provide functional images of the breast tissue, such as MBI, can detect lesions not visible with conventional mammography. Likewise, in some cases it may not be practical to co-register and co-analyze anatomical images from one imaging modality, such as ultrasound systems, and functional images from MBI to further facilitate guided biopsies. For example, one might desire to use anatomical images gathered in substantially real time from an ultrasound imaging system to aid in biopsy guidance coupled with MBI images. However, the logistics of such a process would be quite difficult. For example, US imaging typically requires that the patient be supine and that a handheld scanner be used to scan the breast tissue. In comparison, MBI is usually performed with the patient seated and the breast lightly compressed between the gamma cameras or a camera and paddle. MBI employs light compression forces, for example 10-15 pounds of force, with imaging times in the 5-10 minute range. Because of the differences in patient orientation alone between MBI and ultrasound, the shapes of the breast tissue being imaged with the use of these two modalities are significantly different and, hence, the correlation of an anatomical abnormality with a functional abnormality becomes complicated. Therefore, accurate co-registration of anatomical images from ultrasound and functional information from MBI is not currently possible.
It would therefore be desirable to provide a system and method that provides functional images of the breast and enables real-time feedback of interventional procedures.